In wavelength division multiplexed (WDM) optical communication systems, a number of different optical carrier wavelengths are separately modulated with data to produce modulated optical signals. The modulated optical signals are combined into an aggregate signal and transmitted over an optical transmission path to a receiver. The receiver detects and demodulates the data, e.g. using coherent detection and digital signal processing (DSP).
One type of modulation that may be used in optical communication systems is phase shift keying (PSK). According to different variations of PSK, data is transmitted by modulating the phase of an optical wavelength such that the phase or phase transition of the optical wavelength represents symbols encoding one or more bits. In a binary phase-shift keying (BPSK) modulation scheme, for example, two phases may be used to represent 1 bit per symbol. In a quadrature phase-shift keying (QPSK) modulation scheme, four phases may be used to encode 2 bits per symbol. Other phase shift keying formats include differential phase shift keying (DPSK) formats and variations of PSK and DPSK formats, such as return-to-zero DPSK (RZ-DPSK) and polarization division multiplexed QPSK (PDM-QPSK).
A modulation format, such as QPSK wherein multiple information bits are to be encoded on a single transmitted symbol may be generally referred to as a multi-level modulation format. Multi-level modulation techniques have been used, for example, to allow increased transmission rates and decreased channel spacing, thereby increasing the spectral efficiency (SE) of each channel in a WDM system. One spectrally efficient multi-level modulation format is quadrature amplitude modulation (QAM). In a QAM signal, information is modulated using a combination of phase shift keying and amplitude shift keying, for example, to encode multiple bits per symbol. For any given M2-QAM where M is a positive integer, each symbol represents log2 (M2) bits. For example, a 16-QAM modulation format may be used to encode 4 bits per symbol and 64-QAM may be used to encode 6 bits per symbol. PSK modulation schemes (e.g., BPSK and QPSK) may be referred to as a level of QAM (e.g., 2QAM and 4QAM respectively).
QAM schemes are useful in realizing flexi-rate transponder technology wherein multiple data rates may be achieved over the same bandwidth (or symbol rate) using different SE. In one approach, switching from one SE to another may be achieved by switching between different QAM formats. Although this approach may perform well, it can be limited by a coarse step in achievable bitrates and transmission distances. Switching from 16-QAM to 8-QAM for example will cause a reduction of 25% of the total information bit rate to perhaps provide about an 80% increase in transmission distance. Switching from 16-QAM to QPSK on the other hand will cause a reduction of 50% of the total information bit rate to perhaps provide about 330% increase in transmission distance.